Importance of sunscreen products spreading protocol and substrate roughness for <i>in vitro</i> sun protection factor assessment

<jats:title>Synopsis</jats:title><jats:p>The purpose of this study was to evaluate the impact of substrate roughness and of product spreading method on <jats:italic>in vitro</jats:italic> sun protection factor (SPF) measurement and to define the experimental conditions most appropriate to reach the best level of correlation to <jats:italic>in vivo</jats:italic> SPF. <jats:italic>In vitro</jats:italic> SPF assessment was carried out on 13 products (including different formulation types with SPF from 20 to 75) using various <jats:italic>in vitro</jats:italic> SPF protocols and comparing related predictive potential regarding <jats:italic>in vivo</jats:italic> SPF. In the first part, two spreading methods were compared on two types of PMMA (Polymethyl methacrylate plate with different roughness. The impact of a second spreading step after product drying was also evaluated. From the various investigated parameters, it was shown that (i) a higher roughness (Ra = 4, 5 μm) was preferred for O/W formulations (ii) using a defined sequence of light linear and circular strokes was more adequate than monitoring product spreading in terms of time and pressure (iii) both correlation to <jats:italic>in vivo</jats:italic> SPF and results variability were improved when a second spreading step was added. The altered protocol showed a good predictive potential regarding <jats:italic>in vivo</jats:italic> SPF values for O/W formulations (correlation coefficient 0.92, correlation curve slope 0.98) and coefficient of variation of <jats:italic>in vitro</jats:italic> results (14% of the mean SPF value) close to what is usually obtained <jats:italic>in vivo</jats:italic>. The repeatability of the protocol was also demonstrated. In the second part, we evaluated the impact of PMMA plate pre‐treatment with paraffinum liquidum before spreading the product to get a better correlation between <jats:italic>in vivo</jats:italic> and <jats:italic>in vitro</jats:italic> SPF values for W/O formulations. This allowed us to define a protocol suitable for both O/W and W/O formulations.</jats:p>